1. Field of the Invention
This invention relates to a surface-emitting laser and an optical apparatus formed by using a surface-emitting laser.
2. Description of the Related Art
A two-dimensional array of vertical cavity surface-emitting lasers (VCSELs) can be formed with ease. Then, light can be taken out from each of them in a direction perpendicular to the surface of the semiconductor substrate thereof.
Thus, a plurality of beams of light emitted from such a two-dimensional array can be utilized for parallel processing for the purpose of densification and high speed operations. Therefore, surface-emitting lasers are expected to find various industrial applications.
For example, a high density and high speed printing process can be realized by means of a plurality of beams for an electrophotographic printer when a surface-emitting laser array is employed as exposure light source for the electrophotographic printer.
However, the surface-emitting lasers are required to operate stably in a single transverse mode or in a single longitudinal mode because microspots of laser beam need to be formed stably on a photosensitive drum in the printing process of an electrophotographic printer.
Generally, a surface-emitting laser has a cavity length of several micrometers, which is very short if compared with the cavity length of an edge-emitting laser.
Therefore, the mirror loss of a surface-emitting laser needs to be minimized for laser oscillations. For this reason, a reflector illustrating a high reflectivity (not less than 99%) is employed.
To achieve such a high reflectivity, a distributed Bragg reflector (DBR) formed by alternately laying layers of two different types having different refractive indexes and an optical thickness of λ/4 (λ: oscillation wavelength) is normally employed.
Semiconductor materials that facilitate forming a distributed Bragg reflector and allow current injection are popularly being used for such reflectors.
Additionally, in recent years, there have been proposed techniques of forming a current-confining structure typically by arranging an AlGaAs layer showing an Al composition ratio of 98% in a distributed Bragg reflector and selectively oxidizing it in a hot steam atmosphere and thereby injecting an electric current only into the necessary region.
However, selective oxidation is not desirable from the viewpoint of single transverse mode because it gives rise to higher-order transverse modes as an unnecessarily large difference of refractive index is produced due to the existence of an oxidation layer.
As a countermeasure, a technique of reducing the diameter of the light-emitting region to about 3 μm has been employed so that higher-order transverse modes may not be confined and a single transverse mode oscillation may be realized.
However, the use of such a technique entails a remarkable reduction of the output of each device because of the reduced size of the light-emitting region.
Therefore, techniques of allowing a surface-emitting laser to operate in a single transverse mode, while maintaining a relatively large light-emitting region, by intentionally introducing a loss difference between a fundamental transverse mode and higher-order transverse modes have been discussed.
Photonics Technology Letters, August 2000, Volume: 12, Issue: 8, P939-941; Unold, H. J et al. describes the use of a surface-emitting laser having a long cavity length as a technique of producing a loss difference between a fundamental transverse mode and higher-order transverse modes.
A one-wave cavity that is popularly employed in surface-emitting lasers has an effective cavity length of about 1 to 2 μm. On the other hand, a 2 to 8 μm thick spacer layer is inserted into a surface-emitting laser described in the above-cited document in order to increase the cavity length. As a result, the diffraction loss is increased in higher-order transverse modes and single transverse mode oscillation can be realized with a large light-emitting area (of about 8 μm).